Moving mesh in pose mode is deforming inappropriately
The clavicle bone will by default also move the arm as the arm is its child. Therefore the pauldron should only be weighted to the arm and not the the clavicle. Due to the way armature deformation is calculated, volume loss like this can occur when solving between a number of weights (checking preserve volume on the armature modifier can help, but it can cause other artifacts as well.

Another thing you may want to do is make a separate bone to control the pauldron and use contraints to mix rotation from the clavicle and arm onto the new bone.

(At work so I can't check the blend file, you may also want to check to see if any other bones have influence on the pauldron)